1. Field of the Invention
The present invention relates to an automatic focusing apparatus including a detachably attachable focal length conversion optical system and a focusing method thereof, and in particular, to a method of controlling the automatic focusing apparatus when the focal length conversion optical system is switched.
2. Description of the Related Art
In recent years, a so-called contrast AF method is in the mainstream as to auto-focus (AF) control of a video camera. The contrast AF method detects a sharpness level of an image out of video signals obtained by photoelectrically converting a subject image with an image pickup device and renders it as an AF evaluated value so as to exert control to detect a focus lens position where the AF evaluated value is maximal.
The AF evaluated value generally used is a high-frequency component extracted from the video signal by a band pass filter of a certain band. The AF evaluated value increases as getting closer to a focusing position in the case of shooting an ordinary subject image. And a point at which the level becomes maximal is the focusing position.
There is a TTL phase difference detection method as another AF method.
The TTL phase difference detection method divides a luminous flux from the subject having passed through an exit pupil of an image taking lens into two so as to subject a pair of line sensors to the luminous flux divided in two respectively.
And an out-of-focus amount, that is, a defocus amount of the image taking lens is acquired by detecting a displacement amount of two subject image signals outputted according to a received light amount thereof, that is, a relative positional displacement amount in a direction of division of the luminous flux.
Therefore, once an accumulating operation is performed by a focus detecting sensor, it is possible to obtain the amount and direction for driving the focus lens so as to allow a high-speed focusing operation.
As for other AF methods, there are the methods of measuring the distance to the subject from an ultrasonic propagation velocity by using an ultrasonic sensor, performing triangular surveying by using an infrared sensor and the like. Furthermore, there is also a hybrid AF method combining these AF methods. In the case of the hybrid AF method, the focus lens is driven close to a focused focal point by an internal phase difference detection method and is then further driven to the focusing position accurately by the contrast AF method as in Japanese Patent Application Laid-Open No. H05-64056.
There is switching means used in a TV camera field, whereby a focal length range of the entire system is changed by detaching and attaching a focal length conversion optical system called an extender optical system from and to a part of an optical path of the image taking lens.
In the case of performing an AF with an image taking apparatus by using the contrast AF method, however, an imaging optical path is intercepted in the middle of switching the extender optical system. At this time, an abnormal change occurs to the video signal, and the AF malfunctions according to the abnormal change of the video signal.
Therefore, after completing the switching of the extender optical system, a wasteful AF operation is generated before focusing on the subject based on a correct subject image again. In the meantime, the image immediately after completing the switching of the extender optical system is in an out-of-focus state.
As for a system having a branching optical system placed therein such as the phase difference method, there are the cases where only a magnification ratio of an image provided by image pickup means is changed by switching the extender optical system without changing the magnification ratio of an image provided by focus detecting means. And there is the following problem in this case.
FIG. 10 shows focus detection areas, a subject and an image pickup screen on switching the extender optical system. Reference characters E1 and E2 denote imaging ranges when the extender optical system is 1 time and 2 times respectively, P1 and P2 denote the subjects, and F1 and F2 denote the focus detection areas.
For instance, if the extender optical system is switched from 1 time to 2 times in the case where the focus detection area is set at F2, the focus detection area F2 gets out of the imaging range so that a subject out of the imaging range is focused on. Therefore, the subject in the imaging range is put in the out-of-focus state.